The present invention relates to fuel cells, particularly to small, compact fuel cells, and more particularly to a miniature power source composed of a stack of fuel cells fabricated by combining MEMS and thin film deposition technologies to produce fuel cells with microflow channels, fully-integrated control circuitry, and integrated resistive heaters.
Portable power sources of various types have been under development for many years. A serious need exists for portable power sources with significantly higher power density, longer operating lifetime, and lower cost. Present rechargeable and primary portable power sources have excessive weight, size, and cost with limited mission duration. As an example, batteries covering power range from 1–200 Watts have specific energies ranging from 50–250 Whr/Kg, which represents two to three hours of operation for a variety of commercial and military applications. An alternative power source is the fuel cell which would potentially provide higher performance power sources for portable power applications if the stack structure, packaging, and cell operation were made compatible with scaling down of size and weight.
Fuel cells typically consist of electrolyte materials based on either polymer (proton exchange type) or solid oxide materials, which are sandwiched between electrodes. The fuel cell operates when fuel (usually hydrogen) is delivered to one electrode, and oxygen to the other. By heating the electrode-electrolyte structure, the fuel and oxidant diffuse to the electrode interfaces where an electrochemical reaction occurs, thereby releasing free electrons and ions which conduct across the electrolyte. Typical fuel cells are made from bulk electrode-electrolyte materials which are stacked and manifolded using stainless steel or other packaging which is difficult to miniaturize. These systems are bulky, requiring labor intensive manual assembly, packaging and testing, and in the case of solid oxide materials, typically operate at high temperatures (>600° C.). If the electrode-electrolyte stack can be made very thin and deposited using thin film deposition techniques, the temperature of operation will be significantly lower, and the cost of integration, packaging and manufacturing can be reduced.
Previous efforts at Lawrence Livermore National Laboratory, for example, have demonstrated the synthesis of a thin-film solid-oxide based electrolyte fuel cell. See A. F. Jankowski et al., Mat Res. Soc. Symp. Proc., Vol. 496, pp 155–158, 1998 Material Research Society; and U.S. Pat. No. 5,753,385, issued May 19, 1998 to A. F. Jankowski. In one example, the thin film solid oxide fuel cell (TFSOFC) stack was formed using physical vapor deposition (PVD) techniques. The host substrate used was a silicon wafer covered by a thin layer of silicon nitride. A layer of nickel was first deposited, followed by a layer yttria-stabilized zirconia (YSZ). The conditions during the deposition were adjusted in order to achieve smooth, dense, continuous films, thus, avoiding pinhole formation which could result in electrical shorting through the electrolyte layer. This enables the electrolyte layer to be on the order of 1 μm thick rather than typical thicknesses on the order of >10 μm for bulk solid oxide fuel cells. By thinning the electrolyte layer, the diffusive path for the oxygen ion is shorter and the fuel cell operates at much lower temperatures. A silver electrode layer is deposited on top of the YSZ layer. The deposition conditions of this film are adjusted to create a porous structure so that oxygen can readily diffuse to the electrolyte interface.
The present invention combines an example of thin-film deposition technology, referenced above, with micro-electro-mechanical systems (MEMS) technology to produce a thin-film miniature fuel cell with microflow channels and full-integrated control circuitry, along with integrated resistive heaters for effectively heating the fuel cell such that it will yield and order of magnitude greater power density than any currently known fuel cell. Using this combined technology, thin-film fuel cell stacks can be produced to provide a small, compact miniature power source. The miniature fuel cells of this invention may be either solid oxide or solid polymer or proton exchange membrane electrolyte materials, and may also utilize catalyst layers between the electrodes and the electrolyte.